sisterless A is required for activation of Sex lethal in the Drosophila germline

Both somatic cells and germ cells must establish their correct sexual identity for proper gametogenesis. In Drosophila, sex determination in somatic cells is controlled by the switch gene Sex lethal ( Sxl ), which is activated in females by the presence of two X chromosomes. Though germline sex determination is much less well understood, Sxl is also essential for the female identity in germ cells. Loss of Sxl function in the germline results in ovarian germline tumors, a characteristic of male germ cells developing in a female soma. Further, Sxl expression is sufficient for XY (male) germ cells to produce eggs when transplanted into XX (female) somatic gonads. As in the soma, the presence of two X chromosomes activates Sxl in the germline, but the mechanism for “counting” X chromosomes in the germline is thought to be different from the soma. Here we have explored this mechanism at both cis - and trans- levels. Our data support the model that the Sxl “establishment” promoter ( SxlPE ) is activated in a female-specific manner in the germline, as in the soma, but that the timing of SxlPE activation, and the DNA elements that regulate SxlPE, are different in the germline. Nevertheless, we find that the X chromosome gene sisterless A (sisA), which helps activate Sxl in the soma, is also essential for Sxl activation in the germline. Loss of sisA leads causes of Sxl expression in the germline, and to ovarian tumors and germline loss. These defects can be rescued by Sxl expression, demonstrating that sisA lies upstream of Sxl in germline sex determination. We conclude that sisA acts as an X chromosome counting element in both the soma and the germline, but that additional factors regulating female-specific expression of Sxl in the germline remain to be discovered.